A problem that has been especially acute in clamping rubber hoses to metal couplings in automotive radiator coolant systems has been that of cold temperature leakage. This phenomena occurs after an internal combustion engine has been running long enough to raise the temperature of its coolant and is then turned off long enough to allow the coolant to cool down. As the coolant drops to its lower temperature, the components of the cooling system logically also fall in temperatures. Like most materials, the dimensions of the components increased when they got hotter and then began to decrease as they cooled down. Different materials, however, have different coefficients of expansion, meaning not only that they expand at different rates when they get hot, but also that they contrast at different rates when they cool down. The coefficient of most rubber elastomers is generally on the order of ten times as great as the coefficient of expansion of steel. See Rubber Technology and Manufacture, edited by C. M. Blow, Institution of the Rubber Industry, 1971, at page 294. Thus, of two articles having identical dimensions at a given temperature, one made of steel and one made of rubber, the article made of rubber will contract to a smaller dimension than the article made of steel when the two are simultaneously cooled to the same lower temperature.
Until now, it has been virtually a universal practice among manufacturers to connect the rubber hoses of the cooling system to metal couplings in the cooling system by fitting the hose over a metallic nipple and fitting a clamp over the end portion of the hose that fits over the nipple. The clamp is then tightened down, thereby tightly compressing that portion of the rubber in between the clamp and the nipple. Assuming the clamp has been tightened hard enough, this works well as the temperature and the pressure of the coolant in the system rise. After the engine has been turned off and the temperature of the components begins to fall, however, the metal of the nipple and the rubber in contact with it and under compression begin to contract, although at different rates. Since the rubber is contracting faster than the metal, the part of the rubber hose under compression in contact with the nipple actually begins to shrink away from the metal, permitting the coolant in the system to seep past the connection, thereby resulting in the cold temperature leakage. In recent years, this has been one of the most frequent sources of consumer warranty complaints against automobile manufacturers.
Therefore, a new approach to the problem of engineering a coupling had to be undertaken which got around the problem of metal and rubber having the different coefficients of expansion and separating from each other when they were clamped to each other. Materials science engineers now know that rubber responds differently to temperature depending on whether it is in a compressed state, an unstressed state, or under tension. Rubber under tension has a negative coefficient of thermal expansion, and its Young's modulus will increase as temperature increases, if a given article of rubber is under tension. For instance, if a stretched rubber band is heated, the rubber band will contract if the external load is not changed. See Elements of Material Science and Engineering by Lawrence H. Van Vlack, Addison Wesley 1985 at page 227. Since rubber displays a negative coefficient of expansion when it is under tension, it will, to a certain degree, expand as its temperature is decreased. This results in an interesting phenomena for a piece of rubber hose that has been stretched over a nipple without having a clamp exerting any appreciable compressive force on it. The act of stretching the rubber circumferentially to fit over the nipple has caused it to expand circumferentially and decrease in wall thickness. If the rubber and the metal are now simultaneously cooled to a lower temperature, the metal characteristically contracts, but the rubber will, within a certain range, increase its wall thickness as the temperature decreases, thereby maintaining contact with the metal instead of shrinking away from it.
This phenomena therefore would have applications for the prevention of cold leaks within a certain temperature range. However, use of this application alone would not have much practical value in any system to be put under anything more than nominal working pressure, since the axial forces building up in the hose under increasing pressure would simply blow the hose off the coupling connected in this manner.
A solution to this problem which has been known for a long time is to use a coupling having obverse and reverse tapers on it, fitting the hose over the tapers, and then fitting a similarly tapered ring over the reverse taper of the coupling. See U.S. Pat. No. 756,350, issued Apr. 5, 1904. The reverse taper approach to the problem was found to be useful as a hose mender. However, it was not then known that the height of the apex between the obverse and reverse tapers could be varied in relation to the internal diameter of the hose to be fitted, so as to find an optimal point at which to stretch the hose so as to maximize the negative coefficient of expansion of the rubber within the temperature range that the coolant of a cooling system would rise or fall within. Additionally, it was thought at that time that the external ring had to serve the function of tightly compressing the rubber against the reverse taper of the nipple. Thus, the interior of the ring would not be the same angle as the reverse taper of the nipple, but rather smaller than the reverse taper of the nipple so as to compressively grip the rubber more tightly at one end of the ring. This feature is not necessary at all, because, when pressure is applied to the fluid in the system, axial forces tend to pull the hose away from the coupling, and in so doing, the ring is pulled along by the exterior surface of the hose so as to fit more snugly against the reverse taper as the pressure in the hose increases. Since the connection will get tighter as the pressure increases, the hose will fail due to bursting before it will blow off a coupling made in this manner.